Rapid Solidification And Electrical Resistivity Of Multiphase Alloys

The phase structure, microstructural morphology and electrical resistivity of rapidly solidified monotectic, peritectic and eutectic alloys have been investigated by rapid quenching method. The main results are obtained as follows.The microstructure and phase selection, phase separation behavior and crystal growth characteristic of Fe-Sn and Cu-Pb monotectic alloys are systematically investigated during rapid solidification, the dynamic mechanism of monotectic transformation has been revealed. Under rapid solidifcation conditions, the phase separation is effectively suppressed. The homogeneous microstructure is obtained for various monotectic alloys. For Fe-Sn monotectic alloys, larger cooling rate makes the peritectic transformation generally taking place at elevated temperatures suppressed. The microstructure of Fe-15.6%Sn hypomonotectic alloy consists of single-phase a-Fe solid solution, and that of Fe-40%Sn hypomonotectic and Fe-48.8%Sn monotectic alloy is composed of hexagonal and metastable Fe_1.3Sn,β-Sn and a-Fe phase. Fe-58%Sn hypermonotectic alloy consists of metastableβ-Sn,Fe_1.3Sn and FeSn₂ phase except for producing a few ofα-Fe phase. With the increase of cooling rate, the variety and amounts of metastable phase tends to increase. The microstructure morphology transforms from columnar to equiaxed crystal for monotectic and hypomonotectic alloys. The regular arranged fiber-like microstructure is formed for hypermonotectic alloy.For Cu-Pb monotectic alloys, due to the higher branching ability of Cu-rich phase growing in a dendritic manner in monotectic and hypomonotectic alloys, which will trap the L₂(Pb) droplets in the gap of dendrites and results in the formation of interdendritic fine (Pb) phase. Thus Cu-rich phase distributes in interdendritic spacing to a great extent. The microstructure of hypermonotectic alloy consists of irregular, block-like Cu-rich phase and interdendritic mixture of Pb-rich phase and fine (Cu) dendrite. With the rise of cooling rate, the grain size decreases and the microstructure is refined. The microstructural morphology transforms from coarse dendrite to fine equiaxed grain.The rapid solidification behavior of Co-Cu and Fe-Cu binary alloys has been investigatied systematically. For Co-Cu peritectic alloys, the solubility of Co in (Cu) phase is extended from 7.46% under equilibrium condition to 20%. If C₀>80%Cu, the peritectic phase (Cu) will precipitate from undercooled melt immediately, which results in the formation of single phase (Cu). With the increase of cooling rate, the microstructural morphology transforms from equiaxed to columnar structure. In the composition range of 40～70%Cu, the phase separation is suppressed, solidification microstructure displays obviously tow kinds of crystal zones: fine crystal zone near wheel surface and course crystal zone in free surface. TheαCo and (Cu) phase nucleate competitively and alternatively grow in a manner of dendrite in fine crystal zone. It leads to the mixture structure of tow phase forms. In coarse crystal zone,αCo is the primary phase, a few of Cu-rich phase distributed in the gap ofαCo dendrites, therefore the main phase is theαCo phase in the microstructure.The composition of Fe-Cu alloys with phase separation is in the range of 40～66.4%Cu. Due to the common action of energy and momentum transport, the thermal boundary layer with 160～300μm high and momentum boundary layer with 160²40μm thickness in the bottom of puddle have formed. With the increase of Re number, the thickness of thermal boundary layer monotonously increase. While the thickness of momentum boundary layer slowly decreases at first, then slightly rises and sharply decreases. If Re<1024, due to dominant momentum transport, the liquid flow has remarkable effects on microstructure formation. The separated liquid phase is likely to form the fiber-like microstructure. If Re>1024, since the momentum transport has been suppressed and the energy transport get dominant, the cooling rate of undercooled melt increases, it leads to the formation of equiaxed microstructure.The rapid solidification and microstructural evolution of eutectic alloys have been analyzed deeply. Under larger undercooling or cooling rate, eutectic microstructure is characterized by anomalous eutectic. The competitive nucleation, random branching and interaction growth between tow eutectic phase result in the formation of anomalous eutectic. With the rise of cooling rate, eutectic microstructure is refined markedly, its uniformity increases obviously.The similarities and differences of monotectic, peritectic and eutectic alloys in rapid solidification are compared. For the tow rapid solidification processes——high undercooling and melt quenching methods , though the high undercooling can obtain the larger nucleation undercooling, its cooling rate is relatively low. The cooling rate during melt quenching reaches up to 10⁶K/s. Moreover, the rapid solidification behavior of three kinds of alloys is all sensitive to cooling rate. With the increase of cooling rate, the competitive nucleation and growth tendency of multiphase alloys increase, the microstructure is refined and its uniformity is improved. However, three alloys exhibit different solidification characteristics: anomalous eutectic solidification (eutectic alloy), independent nucleation and growth of peritectic phase (peritectic alloy) and phase separation is suppressed (monotectic alloy). As compared with high undercooling process, the melt quenching method is easy to obtain the fine and uniform rapid solidification microstructure.The relationships between the microstructure and resistivity of three kinds of multiphase alloys have been studied systematically. With the rise of cooling rate, the number of metastable phase increases, which intensifies the scattering of free electrons, leading to the remarkable increase of electrical resistivity. Due to the increase of solubility, crystal defects density and dispersion degree of (Cu) phase as well as microstructure refinement, the resistivity of Cu-Pb peritectic alloys increases remarkably with cooling rate. The increase of cooling rate makes the amount of grain boundary and dislocations in peritectic alloys increase, which intensifies the scattering of free electrons, leading to the remarkable increase of electrical resistivity. Due to the increasing of cooling rate, the solubility of solute in eutectic alloy phase and the amount of crystal defects increase, which intensifies the scattering of free electrons, resulting in the remarkable increase of electrical resistivity. Under the condition that the grain boundary reflection coefficient r approaches 1, the electrical resistivity of rapidly solidified multiphase alloys can be predicted theoretically.